1. Field of the Invention
The present invention relates to a general purpose instrument with a cross-coil type movable magnet to be used, for example, in a fuel meter or a thermometer of a vehicle.
2. Description of Prior Art
In an instrument with a cross-coil type movable magnet including a pointer needle shaft, a movable magnet rotating about the pointer needle shaft is located in a bobbin, and crossed coils crossing each other are wound on the bobbin so that a measured value or the like obtained from an object to be measured is converted into an electric s signal and a current corresponding to the electric current is applied to the crossed coils. This causes magnetic fields to be generated, and the movable magnet is rotated in a direction of the composite magnetic field of the crossed coils, which turns the pointer needle shaft to point to a measured value.
If electric current flow direction through each of the crossed coils is variable, the direction of the composite magnetic field can be arranged with any angle through an arc of 360.degree.. In practice, however, when the instrument is mounted on a vehicle or the like, the directions of application of electric currents to the crossed coils are fixed because a direct-current (DC) power supply, such as a secondary cell or an accumulator, is used as the power supply for the instrument, which limits the range of rotation of the pointer needle shaft to the range of the crossing angle between the crossed coils. To overcome this difficulty, there have been proposed various methods to improve the instrument with a crossed-coil type movable magnet to provide an indication through a wider angle.
One of the proposals is described in Japanese Patent Laid-Open No. 46-6091 in which by use of means for converting an input from a measurement object into a DC signal and for inverting a DC signal exceeding a predetermined value, one of the crossed coils is supplied with an output current that develops one cycle of a substantially sinusoidal signal and the other of the crossed coils is supplied with an output current that develops one cycle of a substantially cosinusoidal signal, as the input quantity increases from the minimum value to the maximum value.
In addition, as described in Japanese Patent Laid-Open No. 50-34255, there exists a method in which an input signal is received and is supplied to a plurality of pulse generators, which generate a plurality of pulse signals to be used to control a switch means disposed between the pulse generators and the crossed coils.
Moreover, in a method described in Japanese Patent Laid-Open No. 51-131357, the crossed coil unit does not comprise two coils crossing each other at a right angle therebetween but includes three coils in respective directions, and an electric signal corresponding to the input quantity is supplied by the use of two adjacent coils as a pair.
The prior art examples described above are respectively attended with disadvantages. Namely, in the method using the means to invert the DC signal and to supply a substantially sinusoidal signal and a substantially cosinusoidal signal to the respective crossed coils, since current having a characteristic of a direct current is supplied, power loss takes place at an output section, which leads to heat generation. Moreover, since e the output characteristic of the pointer needle shaft is fixed, it is difficult to select a display dial or scale e plate most suitable of r an instrument to be adopted.
Furthermore, in the method in which an input is converted into a plurality of pulse signals and the crossed coils are supplied with pulse signals undergoing switch control, since the current flows through the coils intermittently, the pointer needle shaft undesirably oscillates when the input quantity is small.
Next, in the method using three crossed coils, the manufacturing process of the coils is much h more complicated as compared with two crossed coils and hence the cost soars.